turboboost functions by temporarily bumping your clockspeed in predefined intervals to a predefined maximum "boost clock" when your processor is under heavy load primarily to maintain system responsiveness

the first portion of boot has nothing to do with processing power and everything to do with the number and types of devices and chipset features as well as the firmware on your system

so although turboboost is technically enabled early in the boot process by firmware on chipsets/motherboards which support it (which is almost all of them to my knowledge) the boot sequence is IO and firmware but not processing constrained . . . this is why - SSDs - UEFI - PCIe - SATA III \ SAS II - all effectively speed up booting times, as well as disabling all unnecessary devices in firmware (i.e. unused parallel, serial & firewire ports especially)

hypothetically once your OS has been handed over complete control of the system if it has an enormous amount of services to start, tasks to run, drivers to load or is a hypervisor rebooting that needs to restart many VMs then turboboost can help optimize the OS-side of the final booting \\ post-boot process